Definition: Optical fiber consists of thin glass fibres or plastic or any dielectric medium which can carry light signals from one end to the other. Optical fiber refers to the medium and the technology which is related, or you can say that it is connected with the transmission of information in the form of light impulses and this transmission is done along with a glass or plastic wire or fiber. The wires of fiber optic cable can carry much more information than any other conventional copper wire. The typical optical fiber consists of a very narrow strand of glass called the core. Around the core is a concentric layer of glass called the cladding.Optical fibres make use of light to send information through the optical medium.

Evolution or History of an Optical Fibre

The optical fibre was first time demonstrated by Daniel Colladon and Jacques Babinet in Paris in the early 1840s. They made this by refraction of light. After 12 years, John Tyndall included a demonstration on it. In 1960, the laser light was first used as a light source. In 1965, high loss of light discovered. 1n 1970s, the refining of the manufacturing process is there. Later on in the 1980s, optical fibre technology becomes the backbone of long-distance telephone networks in network administration.

An Optical Transmission System has Three essential components

Light source: In such a system a pulse of light indicates bit 1 and the absence of light indicates bit 0. Light source can be an LED or a laser beam.

Transmission medium: Transmission medium is the ultra-thin fiber of glass.

Detector: A detector generates an electrical pulse when the light falls on it.

Structure of Optical Fiber

It has three parts shown in the diagram given below.• Core: It is a central tube as shown in the diagram. It is of skinny size and made up of the optically transparent dielectric medium. It carries the light from the transmitter to the receiver.The diameter of the core varies from 5um to 100um.• Cladding: it is the outer optical material surrounding the core. Its reflective index is lower than the core. It helps to keep the light within the core as it uses the phenomena of total internal reflection.• Buffer coating: It is the plastic coating which protects the fiber. It is made up of silicon rubber. The diameter of the fiber after the coating is 250-300um.

A typical core diameter is 62.5 microns .Typically cladding has a diameter of 125 microns. 100 microwatts power (roughly) a light emitting diode can couple into an optical fiber. Coating the cladding is a protective coating consisting of plastic, it is called the Jacket.

The loss in signal power as light travels down the fiber is called attenuation. An important characteristic of fiber optics is refraction. Refraction is the characteristic of a material to either pass or reflect light. When light passes through a medium, it “bends” as it passes from one medium to the other. An example of this is when we look into a pond of water If the angle of incidence is small, the light rays are reflected and do not pass into the water.

If the angle of incident is great, light passes through the media but is bent or refracted. Optical fibers work on the principle that the core refracts the light and the cladding reflects the light. The core refracts the light and guides the light along its path. The cladding reflects any light back into the core and stops light from escaping through it - it bounds the medium! Fast data transmission rate is an advantage to using fiber optics data transmission.

The working principle of an optical fiberThe working principle of optical fiber cable is the total internal reflection.

Total internal reflection: when a ray of light travels from denser medium to rarer medium in a way such that the angle of incidence is greater than the critical angle, then the ray reflects into the same medium. This phenomenon is known as total internal reflection. Using this phenomenon, the rays in the optical fiber undergo repeated total internal reflections until it emerges out of the other end of the fiber. It does not depend upon the shape of the fiber cable, i.e. the cable can be in bent shape.

Types of Optical Fibers

Single Mode Fiber (SMF)

As its name suggests, in single-mode fiber, only one mode can propagate through the fiber. In this, the diameter of the small core is 5um, and the diameter of cladding is 70um. Also, the difference between the refractive indexes of core and cladding is minimal. There is no dispersion of light in the single-mode fiber. You can also say that there is no degradation of the signal when the light is travelling through the fiber.

The various characteristics of Single mode fiber are:

• The diameter of glass core in single mode fiber is very small ranging from 8 to 10 microns.

• In this mode, light can propagate only in a straight line, without bouncing.

• Fiber glass has lower density (index of refraction) that creates a critical angle close enough to 90° such that the beam propagates in a straight line.

• In this case, propagation of different beams is almost identical and delays are negligible. The beams arrive at destination together and can be recombined with little distortion to the signal.

• Single mode fibers are more expensive and are widely used for long distance communication.

• These types of fibers can transmit data at 50 Gbps for 100 kilometers without amplification.

• It makes use of a laser light source. In this, light pulses are generated by the injection laser diodes (ILD).

•The light is passed through the laser diode in this mode of fiber.

Multimode Fiber (MMF)

• In multimode fiber, multiple beams travel in the core in different paths.

• In multimode fiber, the diameter of core is about 50 microns.

• In this, the diameter of the core is 40um and diameter of cladding is 70um.

•Due to multimode dispersion, there is degradation of signal in this.

• The relative refractive index difference in multi-mode fiber is more significant than the single-mode fiber.• Multi-mode fiber is not suitable for the long-distance communication because of large dispersion and attenuation of the signal.• It can support less bandwidth as comparative to single-mode fiber.• In this, the data rate is up to 1 Gbps.

Step Index Optical Fiber

•A beam of light moves in a straight line in this medium until it reaches the interface of core and the cladding.

• At this interface, the angle of ray is changed due to the change in density.

• In this mode, some beams travel in a straight line through the core and reaches destination without reflection or refraction.

• The beams that strike the interface of core and cladding at and angle smaller than critical angle penetrate the cladding and are lost.

• The beams striking at angle greater than critical angle are reflected back in core and form total internal reflection.

• In this fiber, a ray with smaller angle of incidence requires more bounces thus will take more time to reach the destination whereas the ray with high angle of incidence will require less number of bounces and will reach the destination in lesser time.

• In this, the refractive index of the core and the cladding is constant.• In this, the light rays propagate in the form of meridiognal rays or in zigzag manner.• In this, the rays cross the fiber axis whenever the reflection is made at the core-cladding interface.

Graded Index Optical Fiber

• A graded index fiber has different densities at the core and at the edges. Density is highest at the centre of the core and decreases gradually to its lowest at the edge.

• Because of this difference in densities, different beams refract at different angles into a curve.

• Only the horizontal beams move in a straight line due to constant density at the centre.

• In graded-index fiber, the core has non-uniform refractive index and the cladding has uniform refractive index. The refractive index of the core decreases gradually from the center of the core to the core-cladding interface.• In this, the light rays propagate in the form of skew rays or in helical manner.• In this, the rays don’t cross the fiber axis whenever the reflection is mad at the core-cladding interface.

Advantages of Optical Fiber

1. They are not affected by electrical and magnetic interference as the data travel in form of light.

3. Optical fibers are thin, lighter in weight and small in size as compared to other wired Medias. It is easier to group several optical fibers in one bundle.

4. Glass is more resistant to corrosive materials as compared to copper. Hence can be laid in different environments.

5. In optical fibers, attenuation (loss of signal) is very low. Therefore these fibers can run several kilometers without amplification.

6. Fibers do not leak light and are quite difficult to tap. So they provide security against potential wiretappers.

7. There is no cross-talk problem in optical fibers.

8. They are highly suitable for environments where speed is needed with full accuracy.

9: Photons in fiber do not affect one another (as they have no charge) and are not affected by stray photons outside the fiber. But when electrons move in a wire they affect each other and are themselves affected by electrons outside the wire.

10. The size (diameter) of the optical fibers is very small (it is comparable to the diameter of human hair). Therefore a large number of optical fibers can fit into a cable of small diameter.

11. The material used for the manufacturing of optical fibers is "silica glass". This material is easily available. So the optical fibers cost lower than the cables with metallic conductors.

12. As the light rays have a very high frequency in the GHz range, the bandwidth of the optical tiber is extremely large. This allows transmission of more number of channels. Therefore the information carrying capacity of an optical fiber is much higher than that of a co-axial cable.

Disadvantages of Optical Fiber

2. Being fragile, optical fibers need to be put deep into the land. This causes a lot of installation cost. Also the interface used for these fibers are expensive.

3. Optical fibers are unidirectional for two-way communication, two fibers are required.

4. It is a newer technology and requires skilled people to administer and maintain them.

5. There is requirement of highly skilled staff for the maintenance of the cable. So, the maintenance cost of the optical fiber system is high.

6. It accepts unipolar codes only.7. There is requirement of precise and costly instruments for the optical fiber.8. Jointing of fiber and splicing is a time consuming process.9. It seems to be costly if it is underutilized, i.e. if we don’t make use of optical fiber cable for long time after doing installations.10. Only point-to-point working is possible in optical fiber.

Characteristics of Optical Fiber Cables:

Fiber optic cables have the following characteristics:

1. Fiber optic cabling can provide extremely high bandwidths in the range from 100 mbps to 2 gigabits because light has a much higher frequency than electricity.2. The number of nodes which a fiber optic can support does not depend on its length but on the hub or hubs that connect cables together.3. Fiber optic cable has much lower attenuation and can carry signal to longer distances without using amplifiers and repeaters in between.4. Fiber optic cable is not atlected by EMI effects and can be used in areas where high voltages are passing by.5. The cost of tiber optic cable is more compared to twisted pair and co-axial.6. The installation of fiber optic cables is difficult and tedious.

Applications of Optical Fiber

It has a wider range of applications as it can be used in almost all fields. The fields in which fiber optic cable is used are listed below:

Medical applications:

• In medical field, it is used as a laser beam for surgery.• It is used in endoscopy so that the doctors can see inside the body through the small hole.• It is used as a imaging tool like X-ray imaging.• It is used as light guide.• It is also used for various tests like MRI scans.• It is used in biomedical research and microscopy.

Telecommunication field:

• In this, the optical fiber cable is used for transmitting and receiving.• It has improved the sound quality in voice communications as it make use of light for transmission of data.• It is also resistant to electronic noise. So, it removes disturbance in the telephonic conversations also.• We can have a direct connection between with no echoes by using fiber optic cable.

Networking:

• We can connect servers and users in a variety of network settings by making the use of fiber optic cable.• It increases the speed, quality and accuracy of transmission of data.• Networking between computers has become easier and faster by using fiber optic cables.

Defense/government field:

• Optical systems offers more security for defense purposes as comparative to traditional metal-based systems.• Fiber optic cables are not sensitive to electrical interference. So, leakage of information is less possible in it.• It is also used in other areas like aircrafts, submarines, SONAR, hydrophones for seismic and other underwater applications.

Data storage:

• It is used for faster transfer of data.• It does not allow the leakage of data. So, data is transmitted and received safely by making use of fiber optic cables.

Broadcast/CAV:

• Broadcast or cable companies make use of optical fiber for wiring internet, video or in many other applications for faster transfer of data.• The speed of transferring the data is very high in fiber optic cable as compare to the other cables. They are cheaper also. That’s why the use of fiber optic has grown day by day.

Industrial/commercial:

• In this field, it is often used as a sensory device.• It is also used in wiring, where there is problem of electromagnetic interference.• It is used in those areas where it is difficult to reach by man. In those areas, we make use of fiber optics for imaging.• It is also used in plumbing for the inspection of sewer lines.

Space applications:

• It is highly used in aerospace applications.• It offers the ideal solution for the transmission and receiving of data. It handles our problems up to a large extent.

Mining applications:

• We make use of fiber optical cables in underground mines• Fiber optic cable establishes a network which offers a reliable, high-speed, low-latency, intrinsically safe, interference-proof, flexible communications. It is done so that we can evolve underground mine communication systems.• Optical fiber cable can handle the convergence of data over long distances.

Spectroscopy:

• We make use of optical fire bundles so that we can transmit light from a spectrometer. We transmit this light to a substance which we cannot place inside the spectrometer. We do this to analyse the composition of the substance. A spectrometer analyses the substance by bouncing the light.• So by using optical fiber, we can analyse those substances which are too large to fit inside the spectrometer.

Automotive industry:

• Fiber optic cables are widely used as a purpose for lighting, in the interior parts of the vehicles.• They can transmit signals between the different parts of vehicles at lightning speed.

Lighting and decorations:

• We can make use of fiber optics in the field of decorations also. This is growing day by day.• It provides an easy and economical solution to those projects which offers lighting and decorations. It seems to be attractive also.• Due to this, they are widely used in the decorative areas.